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<div class="g-unit" id="doc-content"><a name="top"></a>

<div id="jd-header" class="guide-header">
  <span class="crumb">
    &nbsp;
    
  </span>
<h1>Application Licensing</h1>
</div>

  <div id="jd-content">

    <div class="jd-descr">
    <div id="qv-wrapper">
<div id="qv">

  <h2>Quickview</h2>
  <ul>
    <li>Licensing lets you protect your application on any device that includes Android Market.</li>
    <li>Your app maintains control of how it enforces its licensing status. </li>
    <li>Adding licensing to an app is straightforward, using the library available through the SDK.</li>
    <li>The service is free and is available to all developers who publish on Android Market. </li>
  </ul>

  <h2>In this document</h2>
  <ol>
    <li><a href="#account">Setting Up A Publisher Account</a></li>
    <li><a href="#dev-setup">Setting Up the Development Environment</a></li>
    <li><a href="#app-integration">Integrating the LVL with Your Application</a>
    <ol>
       <li><a href="#add-library">Including the LVL</a></li>
       <li><a href="#manifest-permission">Adding the licensing permission</a></li>
       <li><a href="#impl-Policy">Implementing a Policy</a></li>
       <li><a href="#impl-Obfuscator">Implementing an Obfuscator</a></li>
       <li><a href="#impl-lc">Checking the license</a></li>
       <li><a href="#impl-DeviceLimiter">Implementing a DeviceLimiter</a></li>
    </ol></li>
    <li><a href="#test-env">Setting Up the Test Environment</a>
    <ol>
       <li><a href="#test-response">Test responses</a></li>
       <li><a href="#test-acct-setup">Test accounts</a></li>
       <li><a href="#acct-signin">Signing in on a device or emulator</a></li>
    </ol></li>
    <li><a href="#app-obfuscation">Obfuscating Your Application</a></li>
    <li><a href="#app-publishing">Publishing a Licensed Application</a></li>
    <li><a href="#support">Where to Get Support</a></li>
  </ol>

  <h2>Appendix</h2>
  <ol>
    <li><a href="#lvl-summary">Summary of LVL Classes and Interfaces</a></li>
    <li><a href="#server-response-codes">Server Response Codes</a></li>
    <li><a href="#extras">Server Response Extras</a></li>
  </ol>

</div>
</div>

<p>Android Market offers a licensing service that lets you enforce licensing
policies for paid applications that you publish through Android Market. With
Android Market Licensing, your applications can query Android Market at run time to
obtain their licensing status for the current user, then allow or disallow
further use as appropriate. </p>

<p>Using the service, you can apply a flexible licensing policy on an
application-by-application basis &mdash; each application can enforce licensing
in the way most appropriate for it. If necessary, an application can apply custom
constraints based on the licensing status obtained from Android Market.
For example, an application can check the licensing status and then apply custom
constraints that allow the user to run it unlicensed for a specific number
of times, or for a specific validity period. An application can also restrict use of the
application to a specific device, in addition to any other constraints. </p>

<p>The licensing service is a secure means of controlling access to your
applications. When an application checks the licensing status, the Market server
signs the licensing status response using a key pair that is uniquely associated
with the publisher account. Your application stores the public key in its
compiled <code>.apk</code> file and uses it to verify the licensing status
response.</p>

<p>Any application that you publish through Android Market can use the Android
Market Licensing service. No special account or registration is needed.
Additionally, because the service uses no dedicated framework APIs, you can add
licensing to any legacy application that uses a minimum API level of 3 or
higher.</p>

<p>To help you add licensing to your application, the Android SDK provides
library sources that you can include in your application project. The
License Verification Library (LVL) handles all of
the licensing-related communication with the Android Market client and the
licensing service. With the LVL integrated, your application can determine its
licensing status for the current user by simply calling a library checker method
and implementing a callback that receives the status.</p>

<p>This document explains how the licensing service works and how to add it to
your application. </p>


<h2 id="overview">Overview</h2>

<p>Android Market Licensing is a network-based service that lets an application
on an Android-powered device query a trusted licensing server, to determine
whether the application is licensed to the current device user. After receiving
the server response, the application can then allow or disallow further use of
the application as needed. In the service, the role of the licensing server is
to provide the license status for the current user; the application itself is
responsible for querying the server and conditionally granting access to the
application. </p>

<h4>Application, Android Market client, and server</h4>

<p>The licensing service is based on the capability of the Android Market server
to determine whether a given user is licensed to use a given application. The
server considers a user licensed if the user is recorded to have purchased the
application, or if the application is available for free. To properly identify
the user and determine the license status, the server requires information about
the application and user &mdash; the application and the Android Market client
work together to assemble the information and pass it to the server. </p>

<p>In the licensing service, an application does not query the licensing server
directly, but instead calls the Android Market client over remote IPC to
initiate a license request. In the license request:</p>

<ul>
<li>The application provides its package name and a nonce that is later used to
validate any response from the server, as well as a callback over which the
response can be returned asynchronously.</li>
<li>The Android Market client, which has greater permissions than the
application, collects the necessary information about the user and the device,
such as the device's primary Google account username, IMSI, and other
information. It then sends the license check request to the server on behalf of
the application.</li>
<li>The server evaluates the request using all available information, attempting
to establish the user's identity to a sufficient level of confidence. The server
then checks the user identity against purchase records for the application and
returns a license response, which the Android Market client returns to the
application over the IPC callback.</li>
</ul>

<p>Notice that during a license check, the application does not manage any
network connections or use any licensing related APIs in the Android platform.
</p>

<div class="figure" style="width:469px">
<img src="../../images/licensing_arch.png" alt=""/>
<p class="img-caption"><strong>Figure 1.</strong> Your application initiates a
license check through the LVL and the Android Market
client, which handles communication with the Market server.</p>
</div>

<h4>License responses secured through public key cryptography</h4>

<p>To ensure the integrity of each license query, the server signs the license
response data using an RSA key pair that is shared exclusively between the
server and the application publisher.</p>

<p>The licensing service generates a single licensing key pair for each
publisher account and exposes the public key in the account's profile page. The
publisher copies the public key and embeds it in the application source code,
then compiles and publishes the <code>.apk.</code> The server retains the
private key internally and uses it to sign license responses for applications
published on that account. </p>

<p>When the application receives a signed response, it uses the embedded public
key to verify the data. The use of public key cryptography in the licensing
service makes it possible for the application to detect responses that have been
tampered with or that are spoofed.</p>

<h4>Use of licensing in your application</h4>

<p>To use licensing in your application, add code to the application to
initiate a license check request and handle the response when it is received.
You can choose when, and how often, you want your application to check its
license and you have full control over how it handles the response, verifies the
signed response data, and enforces access controls. </p>

<p>To simplify the process of adding support for licensing, download and
integrate the Licensing Verification Library, described below. Integration is
straightforward.</p>

<p>When you are finished integrating the LVL, use a test environment
provided by the publisher site to test your application's handling of server
responses. </p>

<p>Finally, publish the application <code>.apk</code> on Market using the
normal process. If you previously used the copy-protection provided by Android
Market, you can remove it from applications that use licensing. </p>

<h4>Licensing Verification Library simplifies implementation</h4>

<p>The Android SDK includes a License Verification Library (LVL) that you can
download and use as the basis for your application's licensing implementation.
The LVL greatly simplifies the process of adding licensing to your application
and helps ensure a more secure, robust implementation for your application. The
LVL provides internal classes that handle most of the standard operations of a
license query, such as contacting Android Market to initiate a license request
and verifying and validating the responses. It also exposes key interfaces that
let you easily plug in your custom code for defining licensing policy and
managing access as needed by your application. The key LVL interfaces are: </p>

<ul>
<li>Policy &mdash; your implementation determines whether to allow access to the
application, based on the license response received from the server and any
other data available (such as from a backend server associated with your
application). The implementation can evaluate the various fields of the license
response and apply other constraints, if needed. The implementation also lets
you manage the handling of license checks that result in errors, such as network
errors.</li>
<li>LicenseCheckerCallback &mdash; your implementation manages access to the
application, based on the result of the Policy's handling of the license
response. Your implementation can manage access in any way needed, including
displaying the license result in the UI or directing the user to purchase the
application (if not currently licensed). </li>
</ul>

<p>To help you get started with a Policy, the LVL provides two fully complete
Policy implementations that you can use without modification or adapt to your
needs:</p>

<ul>
<li><a href="#ServerManagedPolicy">ServerManagedPolicy</a> is a flexible Policy
that uses settings provided by the licensing server to manage response caching
and access to the application while the device is offline (such as when the
user is on an airplane). For most applications, the use of
ServerManagedPolicy is highly recommended. </li>
<li><a href="#StrictPolicy">StrictPolicy</a> is a restrictive Policy that
does not cache any response data and allows the application access <em>only</em>
when the server returns a licensed response.</li>
</ul>

<p>The LVL is available as a downloadable component of the Android SDK. The
component includes both the LVL itself and an example application that shows how
the library should be integrated with your application and how your application
should manage response data, UI interaction, and error conditions. </p>

<p>The LVL sources are provided as an Android <em>library project</em>, which
means that you can maintain a single set of library sources and share them
across multiple applications. A full test environment is also available through
the SDK, so you can develop and test the licensing implementation in your
applications before publishing them, even if you don't have access to a
physical device.</p>

<h4>Requirements and limitations</h4>

<p>Android Market Licensing is designed to let you apply license controls to
applications that you publish through Android Market. The service is not
designed to let you control access to applications that are not published
through Android Market or that are run on devices that do not offer the Android
Market client. </p>

<p>Here are some points to keep in mind as you implement licensing in your
application: </p>

<ul>
<li>Only paid applications published through Market can use the
service. </li>
<li>An application can use the service only if the Android Market client is
installed on its host device and the device is running Android 1.5 (API level 3)
or higher.</li>
<li>To complete a license check, the licensing server must be accessible over
the network. You can implement license caching behaviors to manage access when
there is no network connectivity. </li>
<li>The security of your application's licensing controls ultimately relies on
the design of your implementation itself. The service provides the building
blocks that let you securely check licensing, but the actual enforcement and
handling of the license are factors in your control. By following the best
practices in this document, you can help ensure that your implementation will be
secure.</li>
<li>Adding licensing to an application does not affect the way the application
functions when run on a device that does not offer Android Market.</li>
<li>Licensing is currently for paid apps only, since free apps are considered
licensed for all users. If your application is already published as free, 
you won't be able to upload a new version that uses licensing.</li>
</ul>

<h4>Replacement for Copy Protection</h4>

<p>Android Market Licensing is a flexible, secure mechanism for controlling
access to your applications. It effectively replaces the Copy Protection
mechanism offered on Android Market and gives you wider distribution
potential for your applications. </p>

<ul>
<li>A limitation of the legacy Copy Protection mechanism on Android Market is
that applications using it can be installed only on compatible devices that
provide a secure internal storage environment. For example, a copy-protected
application cannot be downloaded from Market to a device that provides root
access, and the application cannot be installed to a device's SD card. </li>
<li>With Android Market licensing, you can move to a license-based model in
which access is not bound to the characteristics of the host device, but to your
publisher account on Android Market and the licensing policy that you define.
Your application can be installed and controlled on any compatible device on
any storage, including SD card.</li>
</ul>

<p>Although no license mechanism can completely prevent all unauthorized use,
the licensing service lets you control access for most types of normal usage,
across all compatible devices, locked or unlocked, that run Android 1.5 or
higher version of the platform.</p>

<p>The sections below describe how to add Android Market licensing to your
applications. </p>

<h2 id="account">Setting Up a Publisher Account</h2>

<p>Android Market licensing lets you manage access to applications that
users have downloaded from Android Market. To use licensing in an application,
you need to have a publisher account on Android Market so that you can
publish the application to users. </p>

<p>If you don't already have a publisher account, you need to register for one
using your Google account and agree to the terms of service. Once you are
registered, you can upload applications at your convenience and begin debugging
and testing your licensing implementation. For more information about publishing
on Android Market, see <a
href="../../guide/publishing/publishing.html">Publishing Your
Applications</a></p>

<p>To register as an Android Market developer and set up your publisher account,
visit the Android Market publisher site:</p>

<p style="margin-left:2em;"><a
href="http://market.android.com/publish">http://market.android.com/publish</a>
</p>

<p>If you already have a publisher account on Android Market, use your existing
account to set up licensing. You <em>do not</em> need to register for a new
account to support licensing (and doing so is not recommended, especially if you
are adding licensing support to applications that you have already published).
In all cases, if you have published applications, you manage licensing for those
applications through the account on which the applications are published. </p>

<p>Once your publisher account is set up, use the account to:</p>

<ul>
<li>Obtain a public key for licensing</li>
<li>Debug and test an application's licensing implementation, prior to
publishing the application</li>
<li>Publish the applications to which you have added licensing support</li>
</ul>

<h4>Administrative settings for licensing</h4>

<p>Once you are signed into your publisher account, you can manage several
administrative controls for Android Market licensing. The controls are available
in the Edit Profile page, in the "Licensing" panel, shown below. The controls
let you: </p>

<ul>
<li>Set up multiple "test accounts", identified by email address. The licensing
server allows users signed into test accounts on a device or emulator to send
license checks and receive static test responses.</li>
<li>Obtain the account's public key for licensing. When you are implementing
licensing in an application, you must copy the public key string into the
application.</li>
<li>Configure static test responses that the server sends, when it receives a
license check for an application uploaded to the publisher account, from a user
signed in to the publisher account or a test account.</li>
</ul>

<div style="margin-bottom:2em;">

<img src="../../images/licensing_public_key.png" style="text-align:left;margin-bottom:0;" />
<div style="margin:0 2em;padding:0"><strong>Figure 2.</strong> The Licensing
panel of your account's Edit Profile page lets you manage administrative
settings for licensing.</div>
</div>

<p>For more information about how to work with test accounts and static test
responses, see <a href="#test-env">Setting Up a Testing Environment</a>, below.

<h2 id="dev-setup">Setting Up the Development Environment</h2>

<p>Once you've set up your publisher account on Android Market, the next step is
to set up your development environment for licensing. </p>

<p>Setting up your environment for licensing involves these tasks:</p>

<ol>
<li><a href="#download-sdk">Downloading the latest SDK</a>, if you haven't already done so </li>
<li><a href="#runtime-setup">Setting up the runtime environment</a> for development</li>
<li><a href="#download-lvl">Downloading the Market Licensing component</a> into your SDK </li>
<li><a href="#lvl-setup">Setting up the Licensing Verification Library</a></li>
<li><a href="#add-library">Including the LVL library project in your application</a></li>
</ol>

<p>The sections below describe these tasks. When you are done with setup,
you can begin <a href="#app-integration">integrating the LVL into your applications</a>.</p>

<p>To get started, you need to set up a proper runtime environment on which
you can run, debug and test your application's implementation of license
checking and enforcement. </p>


<h3 id="download-sdk">Downloading the latest SDK</h3>

<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Licensing sample application</h2>

<p>To work with Android Market licensing, you need a functioning Android
application to which you can add licensing support. </p>

<p style="margin-top:.5em;">If you are new to Android
and don't yet have a functioning application, the LVL component includes a sample
application that you can set up as a new application project. The sample provides
a complete, working example of how licensing works. For more information, see <a
href="#download-lvl">Downloading the LVL</a>.</p>

</div>
</div>

<p>If you haven't done so, you need to download the Android SDK before you can
develop Android applications. The SDK provides the tools that you need to build
and debug Android applications, including applications that use Android Market
licensing. For complete information, including installation instructions, see
the <a href="../../sdk/index.html">Android SDK</a>. </p>

<p>If you have already installed the SDK, make sure to update the
SDK tools and ADT Plugin to the latest versions. You can update the SDK tools
using the Android SDK and AVD Manager and ADT through <strong>Help</strong> &gt;
<strong>Software Updates...</strong> in Eclipse. </p>

<p>After you've installed the latest SDK and tools, set up your development
environment as described below. </p>


<h3 id="runtime-setup">Setting up the runtime environment</h3>

<p>As described earlier, applications check licensing status not by contacting
the licensing server directly, but by binding to a service provided by the
Android Market application and initiating a license check request. The Android
Market service then handles the direct communication with the licensing server
and finally routes the response back to your application. To debug and test
licensing in your application, you need to set up a runtime environment that
includes the necessary Android Market service, so that your application is able
to send license check requests to the licensing server. </p>

<p>There are two types of runtime environment that you can use: </p>

<ul>
<li>An Android-powered device that includes the Android Market application, or</li>
<li>An Android emulator running the Google APIs Add-on, API level 8 (release 2)
or higher</li>
</ul>

<p>The sections below provide more information. </p>

<h4 id="runtime-device">Running on a device</h4>

<p>You can use an Android-powered device as the runtime environment for
debugging and testing licensing on your application.</p>

<p>The device you use must:</p>

<ul>
<li>Run a standard version of the Android 1.5 or later (API level
3 or higher) platform, <em>and</em> </li>
<li>Run a system image on which the Android Market client application
is preinstalled. </li>
</ul>

<p>If Android Market is not preinstalled in the system image, your application won't
be able to communicate with the Android Market licensing server. </p>

<p>For general information about how to set up a device for use in developing
Android applications, see <a
href="../../guide/developing/device.html">Developing on a Device</a>.</p>

<h4 id="runtime-emulator">Running on an Android emulator</h4>

<p>You can also use an Android emulator as your runtime
environment for debugging and testing licensing.</p>

<p>Because the standard Android platforms provided in the Android SDK <em>do
not</em> include Android Market, you need to download the Google APIs Add-On
platform, API Level 8 (or higher), from the SDK repository. After downloading
the add-on, you need to create an AVD configuration that uses that system image.
</p>

<p>The Google APIs Add-On does not include the full Android Market client.
However, it does provide: </p>

<ul>
<li>An Android Market background service that implements the
ILicensingService remote interface, so that your application can
send license checks over the network to the licensing server. </li>
<li>A set of underlying account services that let you add an a Google account on
the AVD and sign in using your publisher account or test account credentials.
Signing in using your publisher or test account enables you to debug and test
your application without having publish it. For more information see <a
href="#acct-signin">Signing in to an authorized account</a>, below.</li>
</ul>

<p>Several versions of the add-on are available in the SDK repository, but only
<strong>Google APIs Add-On, API 8 (release 2) or higher</strong> version of the
add-on includes the necessary Android Market services. This means that you
cannot use Google APIs Add-On API 7 or lower as a runtime environment for
developing licensing on an emulator.</p>

<div style="margin-bottom:2em;">

<img src="../../images/licensing_gapis_8.png" style="text-align:left;margin-bottom:0;" />
<div style="margin:0 2em;padding:0"><strong>Figure 3.</strong> Google APIs
Add-On, API 8 (release 2) or higher lets you debug and test your licensing
implementation in an emulator.</div>
</div>

<p>To set up an emulator for adding licensing to an application, follow
these steps: </p>

<ol>
  <li>Launch the Android SDK and AVD Manager. </li>
  <li>In the <strong>Available Packages</strong> panel, select and download the
SDK component "Google APIs (Google Inc.) - API Level 8" (or higher) from the SDK
repository, as shown in the figure above.
  <p>When the download is complete, use the Android SDK and AVD Manager to
create a new AVD based on that component, described next.</p></li>
  <li>In the <strong>Virtual
Devices</strong> panel of the Android SDK and AVD Manager, click
<strong>New</strong> and set the configuration details for the new AVD. </li>
  <li>In the dialog that appears, assign a descriptive name to the AVD and then
use the "Target" menu to choose the "Google APIs (Google Inc.) - API Level 8" as
the system image to run on the new AVD. Set the other configuration details as
needed and then click <strong>Create AVD</strong> to finish. The SDK tools
create the new AVD configuration, which then appears in the list of available
Android Virtual Devices.</li>
</ol>

<p>If you are not familiar with AVDs or how to use them, see <a
href="../../guide/developing/devices/index.html">Managing Virtual Devices</a>.</p>

<h4 id="project-update">Updating your project configuration</h4>

<p>After you set up a runtime environment that meets the requirements described
above &mdash; either on an actual device or on an emulator &mdash; make sure to
update your application project or build scripts as needed, so that your compiled
<code>.apk</code> files that use licensing are deployed into that environment.
In particular, if you are developing in Eclipse, make sure that you set up a
Run/Debug Configuration that targets the appropriate device or AVD. </p>

<p>You do not need to make any changes to your application's
build configuration, provided that the project is already configured to compile
against a standard Android 1.5 (API level 3) or higher library. For example:

<ul>
<li>If you have an existing application that is compiled against
the Android 1.5 library, you do not need to make any changes to your
build configuration to support licensing. The build target meets the minimum
requirements for licensing, so you would continue building
against the same version of the Android platform.</li>

<li>Similarly, if you are building against Android 1.5 (API level 3) but
are using an emulator running the Google APIs Add-On API 8 as the application's
runtime environment, there is no need to change your application's build
configuration. </li>
</ul>

<p>In general, adding licensing to an application should have no impact
whatsoever on the application's build configuration.</p>


<h3 id="download-lvl">Downloading the LVL</h3>

<p>The License Verification Library (LVL) is a collection of helper classes that
greatly simplify the work that you need to do to add licensing to your
application. In all cases, we recommend that you download the LVL and use it as
the basis for the licensing implementation in your application.</p>

<p>The LVL is available as a downloadable component of the Android SDK. The
component includes: </p>

<ul>
<li>The LVL sources, stored inside an Android library project. </li>
<li>An example application called "sample" that depends on the LVL library
project. The example illustrates how an application uses the library helper
classes to check and enforce licensing.</li>
</ul>

<p>To download the LVL component into your development environment, use the
Android SDK and AVD Manager. Launch the Android SDK and AVD Manager and then
select the "Market Licensing" component, as shown in the figure below.
Accept the terms and click <strong>Install Selected</strong> to begin the download. </p>

<div style="margin-bottom:2em;">

<img src="../../images/licensing_package.png" style="text-align:left;margin-bottom:0;" />
<div style="margin:0 2em;padding:0"><strong>Figure 4.</strong> The Market
Licensing package contains the LVL and the LVL sample application. </div>
</div>

<p>When the download is complete, the Android SDK and AVD Manager installs both
the LVL library project and the example application into these directories: </p>

<p style="margin-left:2em"><code>&lt;<em>sdk</em>&gt;/extras/google/market_licensing/library/</code>
&nbsp;&nbsp;(the LVL library project)<br />
<code>&lt;<em>sdk</em>&gt;/extras/google/market_licensing/sample/</code>&nbsp;&nbsp;(the example
application)</p>

<p>If you aren't familiar with how to download components into your SDK, see the
<a href="../../sdk/adding-components.html">Adding SDK Components</a>
document. </p>


<h3 id="lvl-setup">Setting Up the Licensing Verification Library</h3>

<p>After downloading the LVL to your computer, you need to set it up in your
development environment, either as an Android library project or by
copying (or importing) the library sources directly into your existing
application package. In general, using the LVL as a library project is recommended,
since it lets you reuse your licensing code across multiple applications and
maintain it more easily over time. Note that the LVL is not designed to be
compiled separately and added to an application as a static .jar file. </p>

<h4>Moving the library sources to a new location</h4>

<p>Because you will be customizing the LVL sources to some extent, you should
make sure to <em>move or copy</em> the library sources (the entire
directory at <code>&lt;<em>sdk</em>&gt;/market_licensing/library/</code>)
to a working directory outside of the SDK. You should then use the relocated
sources as your working set. If you are using a source-code management
system, add and track the sources that are in the working location rather
than those in default location in the SDK. </p>

<p>Moving the library sources is important is because, when you later update the
Market licensing package, the SDK installs the new files to the same location as
the older files. Moving your working library files to a safe location ensures
that your work won't be inadvertently overwritten should you download a new
version of the LVL.</p>

<h4>Creating the LVL as a library project</h4>

<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Working with library projects</h2>

<p>The LVL is provided as an Android library project, which means that you can
share its code and resources across multiple applications. </p>

<p style="margin-top:.5em;">If you aren't familiar with library projects or how
to use them, see <a href="../../guide/developing/projects/index.html#LibraryProjects">
Managing Projects</a>.
</p>
</div>
</div>

<p>The recommended way of using the LVL is setting it up as a new Android
<em>library project</em>. A library project is a type of development project
that holds shared Android source code and resources. Other Android application
projects can reference the library project and, at build time, include its
compiled sources in their <code>.apk</code> files. In the context of licensing,
this means that you can do most of your licensing development once, in a library
project, then include the library sources in your various application projects.
In this way, you can easily maintain a uniform implementation of licensing
across all of your projects and maintain it centrally. </p>

<p>The LVL is provided as a configured library project &mdash; once you have
downloaded it, you can start using it right away. </p>

<p>If you are working in Eclipse with ADT, you need to add the LVL to your
workspace as a new development project, in the same way as you would a new
application project. </p>

<ol>
<li>Use the New Project Wizard to create a new
project from existing sources. Select the LVL's <code>library</code> directory
(the directory containing the library's AndroidManifest.xml file) as the project
root.</li>
<li>When you are creating the library project, you can select any application
name, package, and set other fields as needed. </li>
<li>For the library's build target, select Android 1.5 (API level 3) or higher.</li>
</ol>

<p> When created, the project is
predefined as a library project in its <code>project.properties</code> file, so
no further configuration is needed. </p>

<p>For more information about how to create an application project or work with
library projects in Eclipse, see <a
href="../../guide/developing/projects/projects-eclipse.html">Managing Projects from
Eclipse with ADT</a></p>.

<h4>Copying the LVL sources to your application</h4>

<p>As an alternative to adding the LVL as a library project, you can copy the
library sources directly into your application. To do so, copy (or import) the
LVL's <code>library/src/com</code> directory into your application's
<code>src/</code> directory.</p>

<p>If you add the LVL sources directly to your application, you can skip the
next section and start working with the library, as described in <a
href="#app-integration"></a>.</p>


<h3 id="add-library">Including the LVL library project sources in your
application</h3>

<p>If you want to use the LVL sources as a library project, you need to add a
reference to the LVL library project in your application project properties. This tells
build tools to include the LVL library project sources in your application at
compile time. The process for adding a reference to a library project depends
on your development environment, as described below.</p>

<p> If you are developing in Eclipse with ADT, you should already have added the
library project to your workspace, as described in the previous section. If you
haven't done that already, do it now before continuing. </p>

<p>Next, open the application's project properties window, as shown below.
Select the "Android" properties group and click <strong>Add</strong>, then
choose the LVL library project (com_android_vending_licensing) and click
<strong>OK</strong>. For more information, see 
<a href="../../guide/developing/projects/projects-eclipse.html#SettingUpLibraryProject">
Managing Projects from Eclipse with ADT</a></p>.

<div style="margin-bottom:2em;">

<img src="../../images/licensing_add_library.png" style="text-align:left;margin-bottom:0;" />
<div style="margin:0 2em;padding:0"><strong>Figure 5.</strong> If you are
working in Eclipse with ADT, you can add the LVL library project to your
application from the application's project properties.</div>
</div>

<p>If you are developing using the SDK command-line tools, navigate to the
directory containing your application project and open the
<code>project.properties</code> file. Add a line to the file that specifies the
<code>android.library.reference.&lt;n&gt;</code> key and the path to the
library. For example: </p>

<pre>android.library.reference.1=path/to/library_project</pre>

<p>Alternatively, you can use this command to update the project
properties, including the reference to the library project:</p>

<pre class="no-pretty-print" style="color:black">android update lib-project
--target <em>&lt;target_ID&gt;</em> \
--path <em>path/to/my/app_project</em> \
--library <em>path/to/my/library_project</em>
</pre>

<p>For more information about working with library projects, 
see <a href="../../guide/developing/projects/projects-cmdline.html#SettingUpLibraryProject">
Managing Projects from the Command Line</a></p>.


<h2 id="app-integration">Integrating the LVL with Your Application</h2>

<p>Once you've followed the steps above to set up a publisher account and
development environment, you are ready to begin integrating the LVL with your
application. </p>

<p>Integrating the LVL with your application code involves these tasks:</p>

<ol>
<li><a href="#manifest-permission">Adding the licensing permission</a> your application's manifest.</li>
<li><a href="#impl-Policy">Implementing a Policy</a> &mdash; you can choose one of the full implementations provided in the LVL or create your own.</li>
<li><a href="#impl-Obfuscator">Implementing an Obfuscator</a>, if your Policy will cache any license response data. </li>
<li><a href="#impl-lc">Adding code to check the license</a> in your application's main Activity</li>
<li><a href="#impl-DeviceLimiter">Implementing a DeviceLimiter</a> (optional and not recommended for most applications)</li>
</ol>

<p>The sections below describe these tasks. When you are done with the
integration, you should be able to compile your application successfully and you
can begin testing, as described in <a href="#test-env">Setting Up the Test
Environment</a>.</p>

<p>For an overview of the full set of source files included in the LVL, see <a
href="#lvl-summary">Summary of LVL Classes and Interfaces</a>.</p>


<h3 id="manifest-permission">Adding the licensing permission to your
AndroidManifest.xml</h3>

<p>To use the Android Market application for sending a license check to the
server, your application must request the proper permission,
<code>com.android.vending.CHECK_LICENSE</code>. If your application does
not declare the licensing permission but attempts to initiate a license check,
the LVL throws a security exception.</p>

<p>To request the licensing permission in your application, declare a <a
href="../../guide/topics/manifest/uses-permission-element.html"><code>&lt;uses-permission&gt;</code></a>
element as a child of <code>&lt;manifest&gt;</code>, as follows: </p>

<p style="margin-left:2em;"><code>&lt;uses-permission
android:name="com.android.vending.CHECK_LICENSE"&gt;</code></p>

<p>For example, here's how the LVL sample application declares the permission:
</p>

<pre>&lt;?xml version="1.0" encoding="utf-8"?&gt;

&lt;manifest xmlns:android="http://schemas.android.com/apk/res/android" ..."&gt;
    &lt;!-- Devices &gt;= 3 have version of Android Market that supports licensing. --&gt;
    &lt;uses-sdk android:minSdkVersion="3" /&gt;
    &lt;!-- Required permission to check licensing. --&gt;
    &lt;uses-permission android:name="com.android.vending.CHECK_LICENSE" /&gt;
    ...
&lt;/manifest&gt;
</pre>

<p class="note"><strong>Note:</strong> Currently, you cannot declare the
<code>CHECK_LICENSE</code> permission in the LVL library project's manifest,
because the SDK Tools will not merge it into the manifests of dependent
applications. Instead, you must declare the permission in each dependent
application's manifest. </p>


<h3 id="impl-Policy">Implementing a Policy</h3>

<div class="sidebox-wrapper">
<div class="sidebox">
<h2>ServerManagedPolicy</h2>

<p>The LVL includes a complete Policy implementation called ServerManagedPolicy
that makes use of license-management settings provided by the Android Market
server. </p>

<p style="margin-top:.5em;">Use of ServerManagedPolicy as the basis for your
Policy is strongly recommended. For more information, see <a
href="#ServerManagedPolicy">ServerManagedPolicy</a> section, below.</p>

</div>
</div>

<p>Android Market licensing service does not itself determine whether a
given user with a given license should be granted access to your application.
Rather, that responsibility is left to a Policy implementation that you provide
in your application.</p>

<p>Policy is an interface declared by the LVL that is designed to hold your
application's logic for allowing or disallowing user access, based on the result
of a license check. To use the LVL, your application <em>must</em> provide an
implementation of Policy. </p>

<p>The Policy interface declares two methods, <code>allowAccess()</code> and
<code>processServerResponse()</code>, which are called by a LicenseChecker
instance when processing a response from the license server. It also declares an
enum called <code>LicenseResponse</code>, which specifies the license response
value passed in calls to <code>processServerResponse()</code>. </p>

<ul>
<li><code>processServerResponse()</code> lets you preprocess the raw response
data received from the licensing server, prior to determining whether to grant
access.

<p>A typical implementation would extract some or all fields from the license
response and store the data locally to a persistent store, such as through
<code><a href="../../reference/android/content/SharedPreferences.html">SharedPreferences</a></code> storage, to ensure that the data is
accessible across application invocations and device power cycles. For example,
a Policy would maintain the timestamp of last successful license check, the
retry count, the license validity period, and similar information in a
persistent store, rather than resetting the values each time the application is
launched.</p>

<p>When storing response data locally, the Policy must ensure that the data is
obfuscated (see <a href="#impl-Obfuscator">Implementing an Obfuscator</a>,
below).</p></li>

<li><code>allowAccess()</code> determines whether to grant the user access to
your application, based on any available license response data (from the
licensing server or from cache) or other application-specific information.  For
example, your implementation of <code>allowAccess()</code> could take into
account additional criteria, such as usage or other data retrieved from a
backend server. In all cases, an implementation of <code>allowAccess()</code>
should only return <code>true</code> if the user is licensed to use the
application, as determined by the licensing server, or if there is a transient
network or system problem that prevents the license check from completing. In
such cases, your implementation can maintain a count of retry responses and
provisionally allow access until the next license check is complete.</li>

</ul>

<p>To simplify the process of adding licensing to your application and to
provide an illustration of how a Policy should be designed, the LVL includes
two full Policy implementations that you can use without modification or
adapt to your needs:</p>

<ul>
<li><a href="#ServerManagedPolicy">ServerManagedPolicy</a>, a flexible Policy
that uses server-provided settings and cached responses to manage access across
varied network conditions, and</li>
<li><a href="#StrictPolicy">StrictPolicy</a>, which does not cache any response
data and allows access <em>only</em> if the server returns a licensed
response.</li>
</ul>

<p>For most applications, the use of ServerManagedPolicy is highly
recommended. ServerManagedPolicy is the LVL default and is integrated with
the LVL sample application.</p>


<h4 id="custom-policies">Guidelines for custom policies</h4>

<p>In your licensing implementation, you can use one of the complete policies
provided in the LVL (ServerManagedPolicy or StrictPolicy) or you can create a
custom policy. For any type of custom policy, there are several important design
points to understand and account for in your implementation.</p>

<p>The licensing server applies general request limits to guard against overuse
of resources that could result in denial of service. When an application exceeds
the request limit, the licensing server returns a 503 response, which gets
passed through to your application as a general server error. This means that no
license response will be available to the user until the limit is reset, which
can affect the user for an indefinite period.</p>

<p>If you are designing a custom policy, we recommend that the Policy:
<ol>
<!-- <li>Limits the number of points at which your app calls for a license check
to the minimum. </li> -->
<li>Caches (and properly obfuscates) the most recent successful license response
in local persistent storage.</li>
<li>Returns the cached response for all license checks, for as long as the
cached response is valid, rather than making a request to the licensing server.
Setting the response validity according to the server-provided <code>VT</code>
extra is highly recommended. See <a href="#extras">Server Response Extras</a>
for more information.</li>
<li>Uses an exponential backoff period, if retrying any requests the result in
errors. Note that the Android Market client automatically retries failed
requests, so in most cases there is no need for your Policy to retry them.</li>
<li>Provides for a "grace period" that allows the user to access your
application for a limited time or number of uses, while a license check is being
retried. The grace period benefits the user by allowing access until the next
license check can be completed successfully and it benefits you by placing a
hard limit on access to your application when there is no valid license response
available.</li>
</ol>

<p>Designing your Policy according to the guidelines listed above is critical,
because it ensures the best possible experience for users while giving you
effective control over your application even in error conditions. </p>

<p>Note that any Policy can use settings provided by the licensing server to
help manage validity and caching, retry grace period, and more. Extracting the
server-provided settings is straightforward and making use of them is highly
recommended. See the ServerManagedPolicy implementation for an example of how to
extract and use the extras. For a list of server settings and information about
how to use them, see  <a href="#extras">Server Response Extras</a> in the
Appendix of this document.</p>

<h4 id="ServerManagedPolicy">ServerManagedPolicy</h4>

<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Server Response Extras</h2>

<p>For certain types of licensing responses, the licensing server appends extra
settings to the responses, to help the application manage licensing effectively.
</p>

<p style="margin-top:.5em;">See <a href="#extras">Server Response Extras</a> for
a list of settings and <code>ServerManagedPolicy.java</code> for information
about how a Policy can use the extras.</p>

</div>
</div>

<p>The LVL includes a full and recommended implementation of the Policy
interface called ServerManagedPolicy. The implementation is integrated with the
LVL classes and serves as the default Policy in the library. </p>

<p>ServerManagedPolicy provides all of the handling for license and retry
responses. It caches all of the response data locally in a
<code><a href="../../reference/android/content/SharedPreferences.html">SharedPreferences</a></code> file, obfuscating it with the
application's Obfuscator implementation. This ensures that the license response
data is secure and persists across device power cycles. ServerManagedPolicy
provides concrete implementations of the interface methods
<code>processServerResponse()</code> and <code>allowAccess()</code> and also
includes a set of supporting methods and types for managing license
responses.</p>

<p>Importantly, a key feature of ServerMangedPolicy is its use of
server-provided settings as the basis for managing licensing across an
application's refund period and through varying network and error conditions.
When an application contacts the Android Market server for a license check, the
server appends several settings as key-value pairs in the extras field of certain
license response types. For example, the server provides recommended values for the
application's license validity period, retry grace period, and maximum allowable
retry count, among others. ServerManagedPolicy extracts the values from the
license response in its <code>processServerResponse()</code> method and checks
them in its <code>allowAccess()</code> method. For a list of the server-provided
settings used by ServerManagedPolicy, see <a href="#extras">Server Response
Extras</a> in the Appendix of this document.</p>

<p>For convenience, best performance, and the benefit of using license settings
from the Android Market server, <strong>using ServerManagedPolicy as your
licensing Policy is strongly recommended</strong>. </p>

<p>If you are concerned about the security of license response data that is
stored locally in SharedPreferences, you can use a stronger obfuscation
algorithm or design a stricter Policy that does not store license data. The LVL
includes an example of such a Policy &mdash; see <a
href="#StrictPolicy">StrictPolicy</a> for more information.</p>

<p>To use ServerManagedPolicy, simply import it to your Activity, create an
instance, and pass a reference to the instance when constructing your
LicenseChecker. See <a href="#lc-lcc">Instantiate LicenseChecker and
LicenseCheckerCallback</a> for more information. </p>

<h4 id="StrictPolicy">StrictPolicy</h4>

<p>The LVL includes an alternative full implementation of the Policy interface
called StrictPolicy. The StrictPolicy implementation provides a more restrictive
Policy than ServerManagedPolicy, in that it does not allow the user to access
the application unless a license response is received from the server at the
time of access that indicates that the user is licensed.</p>

<p>The principal feature of StrictPolicy is that it does not store <em>any</em>
license response data locally, in a persistent store. Because no data is stored,
retry requests are not tracked and cached responses can not be used to fulfill
license checks. The Policy allows access only if:</p>

<ul>
<li>The license response is received from the licensing server, and </li>
<li>The license response indicates that the user is licensed to access the
application. </li>
</ul>

<p>Using StrictPolicy is appropriate if your primary concern is to ensure that,
in all possible cases, no user will be allowed to access the application unless
the user is confirmed to be licensed at the time of use. Additionally, the
Policy offers slightly more security than ServerManagedPolicy &mdash; since
there is no data cached locally, there is no way a malicious user could tamper
with the cached data and obtain access to the application.</p>

<p>At the same time, this Policy presents a challenge for normal users, since it
means that they won't be able to access the application when there is no network
(cell or wi-fi) connection available. Another side-effect is that your
application will send more license check requests to the server, since using a
cached response is not possible.</p>

<p>Overall, this policy represents a tradeoff of some degree of user convenience
for absolute security and control over access. Consider the tradeoff carefully
before using this Policy.</p>

<p>To use StrictPolicy, simply import it to your Activity, create an instance,
and pass a reference to it when constructing your LicenseChecker. See
<a href="#lc-lcc">Instantiate LicenseChecker and LicenseCheckerCallback</a>
for more information. </p>

<h3 id="impl-Obfuscator">Implementing an Obfuscator</h3>

<div class="sidebox-wrapper">
<div class="sidebox">
<h2>AESObfuscator</h2>

<p>The LVL includes a full Obfuscator implementation in the
<code>AESObfuscator.java</code> file. The Obfuscator uses AES encryption to
obfuscate/unobfuscate data. If you are using a Policy (such as
ServerManagedPolicy) that caches license response data, using AESObfuscator as
basis for your Obfuscator implementation is highly recommended. </p>

</div>
</div>

<p>A typical Policy implementation needs to save the license response data for
an application to a persistent store, so that it is accessible across
application invocations and device power cycles.  For example, a Policy would
maintain the timestamp of the last successful license check, the retry count,
the license validity period, and similar information in a persistent store,
rather than resetting the values each time the application is launched. The
default Policy included in the LVL, ServerManagedPolicy, stores license response
data in a <code><a href="../../reference/android/content/SharedPreferences.html">SharedPreferences</a></code> instance, to ensure that the
data is persistent. </p>

<p>Because the Policy will use stored license response data to determine whether
to allow or disallow access to the application, it <em>must</em> ensure that any
stored data is secure and cannot be reused or manipulated by a root user on a
device. Specifically, the Policy must always obfuscate the data before storing
it, using a key that is unique for the application and device. Obfuscating using
a key that is both application-specific and device-specific is critical, because
it prevents the obfuscated data from being shared among applications and
devices.</p>

<p>The LVL assists the application with storing its license response data in a
secure, persistent manner. First, it provides an Obfuscator
interface that lets your application supply the obfuscation algorithm of its
choice for stored data. Building on that, the LVL provides the helper class
PreferenceObfuscator, which handles most of the work of calling the
application's Obfuscator class and reading and writing the obfuscated data in a
SharedPreferences instance. </p>

<p>The LVL provides a full Obfuscator implementation called
AESObfuscator that uses AES encryption to obfuscate data. You can
use AESObfuscator in your application without modification or you
can adapt it to your needs. For more information, see the next section.</p>


<h4 id="AESObfuscator">AESObfuscator</h4>

<p>The LVL includes a full and recommended implementation of the Obfuscator
interface called AESObfuscator. The implementation is integrated with the
LVL sample application and serves as the default Obfuscator in the library. </p>

<p>AESObfuscator provides secure obfuscation of data by using AES to
encrypt and decrypt the data as it is written to or read from storage.
The Obfuscator seeds the encryption using three data fields provided
by the application: </p>

<ol>
<li>A salt &mdash; an array of random bytes to use for each (un)obfuscation. </li>
<li>An application identifier string, typically the package name of the application.</li>
<li>A device identifier string, derived from as many device-specific sources
as possible, so as to make it as unique.</li>
</ol>

<p>To use AESObfuscator, first import it to your Activity. Declare a private
static final array to hold the salt bytes and initialize it to 20 randomly
generated bytes.</p>

<pre>    ...
    // Generate 20 random bytes, and put them here.
    private static final byte[] SALT = new byte[] {
     -46, 65, 30, -128, -103, -57, 74, -64, 51, 88, -95,
     -45, 77, -117, -36, -113, -11, 32, -64, 89
     };
    ...
</pre>

<p>Next, declare a variable to hold a device identifier and generate a value for
it in any way needed. For example, the sample application included in the LVL
queries the system settings for the
<code>android.Settings.Secure.ANDROID_ID</code>, which is unique to each device.
</p>

<p>Note that, depending on the APIs you use, your application might need to
request additional permissions in order to acquire device-specific information.
For example, to query the <code><a href="../../reference/android/telephony/TelephonyManager.html">TelephonyManager</a></code> to obtain
the device IMEI or related data, the application will also need to request the
<code>android.permission.READ_PHONE_STATE</code> permission in its manifest.</p>

<p>Before requesting new permissions for the <em>sole purpose</em> of acquiring
device-specific information for use in your Obfuscator, consider
how doing so might affect your application or its filtering on Android Market
(since some permissions can cause the SDK build tools to add
the associated <code>&lt;uses-feature&gt;</code>).</p>

<p>Finally, construct an instance of AESObfuscator, passing the salt,
application identifier, and device identifier. You can construct the instance
directly, while constructing your Policy and LicenseChecker. For example:</p>

<pre>    ...
    // Construct the LicenseChecker with a Policy.
    mChecker = new LicenseChecker(
        this, new ServerManagedPolicy(this,
            new AESObfuscator(SALT, getPackageName(), deviceId)),
        BASE64_PUBLIC_KEY  // Your public licensing key.
        );
    ...
</pre>

<p>For a complete example, see MainActivity in the LVL sample application.</p>


<h3 id="impl-lc">Checking the license from your application's main Activity</h3>

<p>Once you've implemented a Policy for managing access to your application, the
next step is to add a license check to your application, which initiates a query
to the licensing server if needed and manages access to the application based on
the license response. All of the work of adding the license check and handling
the response takes place in your main <code><a href="../../reference/android/app/Activity.html">Activity</a></code> source file.
</p>

<p>To add the license check and handle the response, you must:</p>

<ol>
    <li><a href="#imports">Add imports</a></li>
    <li><a href="#lc-impl">Implement LicenseCheckerCallback</a> as a private inner class</li>
    <li><a href="#thread-handler">Create a Handler</a> for posting from LicenseCheckerCallback to the UI thread</li>
    <li><a href="#lc-lcc">Instantiate LicenseChecker</a> and LicenseCheckerCallback</li>
    <li><a href="#check-access">Call checkAccess()</a> to initiate the license check</li>
    <li><a href="#account-key">Embed your public key</a> for licensing</li>
    <li><a href="#handler-cleanup">Call your LicenseChecker's onDestroy() method</a> to close IPC connections.</li>
</ol>

<p>The sections below describe these tasks. </p>

<h4 id="lc-overview">Overview of license check and response</h4>

<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Example: MainActivity</h2>

<p>The sample application included with the LVL provides a full example of how
to initiate a license check and handle the result, in the
<code>MainActivity.java</code> file.</p>

</div>
</div>

<p>In most cases, you should add the license check to your application's main
<code><a href="../../reference/android/app/Activity.html">Activity</a></code>, in the <code>onCreate()</code> method. This
ensures that when the user launches your application directly, the license check
will be invoked immediately. In some cases, you can add license checks in other
locations as well. For example, if your application includes multiple Activity
components that other applications can start by <code><a href="../../reference/android/content/Intent.html">Intent</a></code>,
you could add license checks in those Activities.</p>

<p>A license check consists of two main actions: </p>

<ul>
<li>A call to a method to initiate the license check &mdash; in the LVL, this is
a call to the <code>checkAccess()</code> method of a LicenseChecker object that
you construct.</li>
<li>A callback that returns the result of the license check. In the LVL, this is
a <code>LicenseCheckerCallback</code> interface that you implement. The
interface declares two methods, <code>allow()</code> and
<code>dontAllow()</code>, which are invoked by the library based on to the
result of the license check. You implement those two methods with whatever logic
you need, to allow or disallow the user access to your application. Note that
these methods do not determine <em>whether</em> to allow access &mdash; that
determination is the responsibility of your Policy implementation. Rather, these
methods simply provide the application behaviors for <em>how</em> to allow and
disallow access (and handle application errors).</li>
</ul>

<div style="margin-bottom:2em;">

<img src="../../images/licensing_flow.png" style="text-align:left;margin-bottom:0;margin-left:3em;" />
<div style="margin:.5em 0 1.5em 2em;padding:0"><strong>Figure 6.</strong> Overview of a
typical license check interaction.</div>
</div>

<p>The diagram above illustrates how a typical license check takes place: </p>

<ol>
<li>Code in the application's main Activity instantiates LicenseCheckerCallback
and LicenseChecker objects. When constructing LicenseChecker, the code passes in
<code><a href="../../reference/android/content/Context.html">Context</a></code>, a Policy implementation to use, and the
publisher account's public key for licensing as parameters. </li>
<li>The code then calls the <code>checkAccess()</code> method on the
LicenseChecker object. The method implementation calls the Policy to determine
whether there is a valid license response cached locally, in
<code><a href="../../reference/android/content/SharedPreferences.html">SharedPreferences</a></code>.
<ul>
<li>If so, the <code>checkAccess()</code> implementation calls
<code>allow()</code>.</li>
<li>Otherwise, the LicenseChecker initiates a license check request that is sent
to the licensing server.</li>
</ul>
</li>
<li>When a response is received, LicenseChecker creates a LicenseValidator that
verifies the signed license data and extracts the fields of the response, then
passes them to your Policy for further evaluation.
  <ul>
    <li>If the license is valid, the Policy caches the response in
SharedPreferences and notifies the validator, which then calls the
<code>allow()</code> method on the LicenseCheckerCallback object. </li>
    <li>If the license not valid, the Policy notifies the validator, which calls
the <code>dontAllow()</code> method on LicenseCheckerCallback. </li>
  </ul>
</li>
<li>In case of a recoverable local or server error, such as when the network is
not available to send the request, LicenseChecker passes a RETRY response to
your Policy's <code>processServerResponse()</code> method. </li>
<li>In case of a application error, such as when the application attempts to
check the license of an invalid package name, LicenseChecker passes an error
response to the LicenseCheckerCallback's  <code>applicationError()</code>
method. </li>
</ol>

<p>Note that, in addition to initiating the license check and handling the
result, which are described in the sections below, your application also needs
to provide a <a href="#impl-Policy">Policy implementation</a> and, if the Policy
stores response data (such as ServerManagedPolicy), an <a
href="#impl-Obfuscator">Obfuscator</a> implementation. </p>


<h4 id="imports">Add imports</h4>

<p>First, open the class file of the application's main Activity and import
LicenseChecker and LicenseCheckerCallback from the LVL package.</p>

<pre>    import com.android.vending.licensing.LicenseChecker;
    import com.android.vending.licensing.LicenseCheckerCallback;</pre>

<p>If you are using the default Policy implementation provided with the LVL,
ServerManagedPolicy, import it also, together with the AESObfuscator. If you are
using a custom Policy or Obfuscator, import those instead. </p>

<pre>    import com.android.vending.licensing.ServerManagedPolicy;
    import com.android.vending.licensing.AESObfuscator;</pre>

<h4 id="lc-impl">Implement LicenseCheckerCallback as a private inner class</h4>

<p>LicenseCheckerCallback is an interface provided by the LVL for handling
result of a license check. To support licensing using the LVL, you must
implement LicenseCheckerCallback and
its methods to allow or disallow access to the application.</p>

<p>The result of a license check is always a call to one of the
LicenseCheckerCallback methods, made based on the validation of the response
payload, the server response code itself, and any additional processing provided
by your Policy. Your application can implement the methods in any way needed. In
general, it's best to keep the methods simple, limiting them to managing UI
state and application access. If you want to add further processing of license
responses, such as by contacting a backend server or applying custom constraints,
you should consider incorporating that code into your Policy, rather than
putting it in the LicenseCheckerCallback methods. </p>

<p>In most cases, you should declare your implementation of
LicenseCheckerCallback as a private class inside your application's main
Activity class. </p>

<p>Implement the <code>allow()</code> and <code>dontAllow()</code> methods as
needed. To start with, you can use simple result-handling behaviors in the
methods, such as displaying the license result in a dialog. This helps you get
your application running sooner and can assist with debugging. Later, after you
have determined the exact behaviors you want, you can add more complex handling.
</p>

<p>Some suggestions for handling unlicensed responses in
<code>dontAllow()</code> include: </p>

<ul>
<li>Display a "Try again" dialog to the user, including a button to initiate a
new license check. </li>
<li>Display a "Purchase this application" dialog, including a button that
deep-links the user to the application's details page on Market, from which the
use can purchase the application. For more information on how to set up such
links, see <a
href="../../guide/publishing/publishing.html#marketintent">Using Intents to
Launch the Market Application on a Device</a>. </li>
<li>Display a Toast notification that indicates that the features of the
application are limited because it is not licensed. </li>
</ul>

<p>The example below shows how the LVL sample application implements
LicenseCheckerCallback, with methods that display the license check result in a
dialog. </p>

<pre>    private class MyLicenseCheckerCallback implements LicenseCheckerCallback {
        public void allow() {
            if (isFinishing()) {
                // Don't update UI if Activity is finishing.
                return;
            }
            // Should allow user access.
            displayResult(getString(R.string.allow));
        }

        public void dontAllow() {
            if (isFinishing()) {
                // Don't update UI if Activity is finishing.
                return;
            }
            displayResult(getString(R.string.dont_allow));
            // Should not allow access. An app can handle as needed,
            // typically by informing the user that the app is not licensed
            // and then shutting down the app or limiting the user to a
            // restricted set of features.
            // In this example, we show a dialog that takes the user to Market.
            showDialog(0);
        }
    }
</pre>

<p>Additionally, you should implement the <code>applicationError()</code>
method, which the LVL calls to let your application handle errors that are not
retryable. For a list of such errors, see <a
href="#server-response-codes">Server Response Codes</a> in the Appendix of this
document. You can implement the method in any way needed. In most cases, the
method should log the error code and call <code>dontAllow()</code>.</p>

<h4 id="thread-handler">Create a Handler for posting from LicenseCheckerCallback
to the UI thread</h4>

<p>During a license check, the LVL passes the request to the Android Market
application, which handles communication with the licensing server. The LVL
passes the request over asynchronous IPC (using <code><a href="../../reference/android/os/Binder.html">Binder</a></code>) so
the actual processing and network communication do not take place on a thread
managed by your application. Similarly, when the Android Market application
receives the result, it invokes a  callback method over IPC, which in turn
executes in an IPC thread pool in your application's process.</p>

<p>The LicenseChecker class manages your application's IPC communication with
the Android Market application, including the call that sends the request and
the callback that receives the response. LicenseChecker also tracks open license
requests and manages their timeouts. </p>

<p>So that it can handle timeouts properly and also process incoming responses
without affecting your application's UI thread, LicenseChecker spawns a
background thread at instantiation. In the thread it does all processing of
license check results, whether the result is a response received from the server
or a timeout error. At the conclusion of processing, the LVL calls your
LicenseCheckerCallback methods from the background thread. </p>

<p>To your application, this means that:</p>

<ol>
<li>Your LicenseCheckerCallback methods will be invoked, in many cases, from a
background thread.</li>
<li>Those methods won't be able to update state or invoke any processing in the
UI thread, unless you create a Handler in the UI thread and have your callback
methods post to the Handler.</li>
</ol>

<p>If you want your LicenseCheckerCallback methods to update the UI thread,
instantiate a <code><a href="../../reference/android/os/Handler.html">Handler</a></code> in the main Activity's 
<code><a href="../../reference/android/app/Activity.html#onCreate(android.os.Bundle)">onCreate()</a></code> method,
as shown below. In this example, the LVL sample application's
LicenseCheckerCallback methods (see above) call <code>displayResult()</code> to
update the UI thread through the Handler's
<code><a href="../../reference/android/os/Handler.html#post(java.lang.Runnable)">post()</a></code> method.</p>

<pre>private Handler mHandler;

    &#64;Override
    public void onCreate(Bundle savedInstanceState) {
        ...
        mHandler = new Handler();
    }
</pre>

<p>Then, in your LicenseCheckerCallback methods, you can use Handler methods to
post Runnable or Message objects to the Handler. Here's how the sample
application included in the LVL posts a Runnable to a Handler in the UI thread
to display the license status.</p>

<pre>    private void displayResult(final String result) {
        mHandler.post(new Runnable() {
            public void run() {
                mStatusText.setText(result);
                setProgressBarIndeterminateVisibility(false);
                mCheckLicenseButton.setEnabled(true);
            }
        });
    }
</pre>

<h4 id="lc-lcc">Instantiate LicenseChecker and LicenseCheckerCallback</h4>

<p>In the main Activity's
<code><a href="../../reference/android/app/Activity.html#onCreate(android.os.Bundle)">onCreate()</a></code> method,
create private instances of LicenseCheckerCallback and LicenseChecker. You must
instantiate LicenseCheckerCallback first, because you need to pass a reference
to that instance when you call the contructor for LicenseChecker. </p>

<p>When you instantiate LicenseChecker, you need to pass in these parameters:</p>

<ul>
<li>The application <code><a href="../../reference/android/content/Context.html">Context</a></code></li>
<li>A reference to the Policy implementation to use for the license check. In
most cases, you would use the default Policy implementation provided by the LVL,
ServerManagedPolicy. </li>
<li>The String variable holding your publisher account's public key for
licensing. </li>
</ul>

<p>If you are using ServerManagedPolicy, you won't need to access the class
directly, so you can instantiate it in the LicenseChecker constructor,
as shown in the example below. Note that you need to pass a reference to a new
Obfuscator instance when you construct ServerManagedPolicy.</p>

<p>The example below shows the instantiation of LicenseChecker and
LicenseCheckerCallback from the <code>onCreate()</code> method of an Activity
class. </p>

<pre>public class MainActivity extends Activity {
    ...
    private LicenseCheckerCallback mLicenseCheckerCallback;
    private LicenseChecker mChecker;

    &#64;Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ...
        // Construct the LicenseCheckerCallback. The library calls this when done.
        mLicenseCheckerCallback = new MyLicenseCheckerCallback();

        // Construct the LicenseChecker with a Policy.
        mChecker = new LicenseChecker(
            this, new ServerManagedPolicy(this,
                new AESObfuscator(SALT, getPackageName(), deviceId)),
            BASE64_PUBLIC_KEY  // Your public licensing key.
            );
        ...
    }
}
</pre>


<p>Note that LicenseChecker calls the LicenseCheckerCallback methods from the UI
thread <em>only</em> if there is valid license response cached locally. If the
license check is sent to the server, the callbacks always originate from the
background thread, even for network errors. </p>


<h4 id="check-access">Call checkAccess() to initiate the license check</h4>

<p>In your main Activity, add a call to the <code>checkAccess()</code> method of the
LicenseChecker instance. In the call, pass a reference to your
LicenseCheckerCallback instance as a parameter. If you need to handle any
special UI effects or state management before the call, you might find it useful
to call <code>checkAccess()</code> from a wrapper method. For example, the LVL
sample application calls <code>checkAccess()</code> from a
<code>doCheck()</code> wrapper method:</p>

<pre>    &#64;Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ...
        // Call a wrapper method that initiates the license check
        doCheck();
        ...
    }
    ...
    private void doCheck() {
        mCheckLicenseButton.setEnabled(false);
        setProgressBarIndeterminateVisibility(true);
        mStatusText.setText(R.string.checking_license);
        mChecker.checkAccess(mLicenseCheckerCallback);
    }
</pre>


<h4 id="account-key">Embed your public key for licensing</h4>

<p>For each publisher account, the Android Market service automatically
generates a  2048-bit RSA public/private key pair that is used exclusively for
licensing. The key pair is uniquely associated with the publisher account and is
shared across all applications that are published through the account. Although
associated with a publisher account, the key pair is <em>not</em> the same as
the key that you use to sign your applications (or derived from it).</p>

<p>The Android Market publisher site exposes the public key for licensing to any
developer signed in to the publisher account, but it keeps the private key
hidden from all users in a secure location. When an application requests a
license check for an application published in your account, the licensing server
signs the license response using the private key of your account's key pair.
When the LVL receives the response, it uses the public key provided by the
application to verify the signature of the license response. </p>

<p>To add licensing to an application, you must obtain your publisher account's
public key for licensing and copy it into your application. Here's how to find
your account's public key for licensing:</p>

<ol>
<li>Go to the Android Market <a
href="http://market.android.com/publish">publisher site</a> and sign in.
Make sure that you sign in to the account from which the application you are
licensing is published (or will be published). </li>
<li>In the account home page, locate the "Edit profile" link and click it. </li>
<li>In the Edit Profile page, locate the "Licensing" pane, shown below. Your
public key for licensing is given in the "Public key" text box. </li>
</ol>

<p>To add the public key to your application, simply copy/paste the key string
from the text box into your application as the value of the String variable
<code>BASE64_PUBLIC_KEY</code>. When you are copying, make sure that you have
selected the entire key string, without omitting any characters. </p>

<p>Here's an example from the LVL sample application:</p>

<pre>    public class MainActivity extends Activity {
        private static final String BASE64_PUBLIC_KEY = "MIIBIjANBgkqhkiG ... "; //truncated for this example
    ...
    }
</pre>

<h4 id="handler-cleanup">Call your LicenseChecker's onDestroy() method
to close IPC connections</h4>

<p>Finally, to let the LVL clean up before your application
<code><a href="../../reference/android/content/Context.html">Context</a></code> changes, add a call to the LicenseChecker's
<code>onDestroy()</code> method from your Activity's
<code><a href="../../reference/android/app/Activity.html#onDestroy()">onDestroy()</a></code> implementation. The call causes the
LicenseChecker to properly close any open IPC connection to the Android Market
application's ILicensingService and removes any local references to the service
and handler.</p>

<p>Failing to call the LicenseChecker's <code>onDestroy()</code> method
can lead to problems over the lifecycle of your application. For example, if the
user changes screen orientation while a license check is active, the application
<code><a href="../../reference/android/content/Context.html">Context</a></code> is destroyed. If your application does not
properly close the LicenseChecker's IPC connection, your application will crash
when the response is received. Similarly, if the user exits your application
while a license check is in progress,  your application will crash when the
response is received, unless it has properly called the
LicenseChecker's <code>onDestroy()</code> method to disconnect from the service.
</p>

<p>Here's an example from the sample application included in the LVL, where
<code>mChecker</code> is the LicenseChecker instance:</p>

<pre>    &#64;Override
    protected void onDestroy() {
        super.onDestroy();
        mChecker.onDestroy();
        ...
    }
</pre>

<p>If you are extending or modifying LicenseChecker, you might also need to call
the LicenseChecker's <code>finishCheck()</code> method, to clean up any open IPC
connections.</p>

<h3 id="impl-DeviceLimiter">Implementing a DeviceLimiter</h3>

<p>In some cases, you might want your Policy to limit the number of actual
devices that are permitted to use a single license. This would prevent a user
from moving a licensed application onto a number of devices and using the
application on those devices under the same account ID. It would also prevent a
user from "sharing" the application by providing the account information
associated with the license to other individuals, who could then sign in to that
account on their devices and access the license to the application. </p>

<p>The LVL supports per-device licensing by providing a
<code>DeviceLimiter</code> interface, which declares a single method,
<code>allowDeviceAccess()</code>. When a LicenseValidator is handling a response
from the licensing server, it calls <code>allowDeviceAccess()</code>, passing a
user ID string extracted from the response.</p>

<p>If you do not want to support device limitation, <strong>no work is
required</strong> &mdash; the LicenseChecker class automatically uses a default
implementation called NullDeviceLimiter. As the name suggests, NullDeviceLimiter
is a "no-op" class whose <code>allowDeviceAccess()</code> method simply returns
a <code>LICENSED</code> response for all users and devices. </p>

<div style="border-left:4px solid #FFCF00;margin:1em;padding: 0 0 0 .5em">
<p><strong>Caution:</strong> Per-device licensing is <em>not recommended for
most applications</em> because:</p>
<ul>
<li>It requires that you provide a backend server to manage a users and devices
mapping, and </li>
<li>It could inadvertently result in a user being denied access to an
application that they have legitimately purchased on another device.</li>
</ul>
</div>


<h2 id="test-env">Setting Up the Testing Environment</h2>

<p>The Android Market publisher site provides configuration tools that let you
and others test licensing on your application before it is published. As you are
implementing licensing, you can make use of the publisher site tools to test
your application's Policy and handling of different licensing responses and
error conditions.</p>

<p>The main components of the test environment for licensing include: </p>

<ul>
<li>A "Test response" configuration in your publisher account that lets you
set the static licensing response returned, when the server processes a
license check for an application uploaded to the publisher account, from a user
signed in to the publisher account or a test account.</li>
<li>An optional set of test accounts that will receive the static test
response when they check the license of an application that you have uploaded
(regardless whether the application is published or not).</li>
<li>A runtime environment for the application that includes the Android Market
application or Google APIs Add-On, on which the user is signed in to the
publisher account or one of the test accounts.</li>
</ul>

<p>Setting up the test environment properly involves:</p>

<ol>
<li><a href="#test-response">Setting static test responses</a> that are returned by the licensing server.</li>
<li><a href="#test-acct-setup">Setting up test accounts</a> as needed.</li>
<li><a href="#acct-signin">Signing in</a> properly to an emulator or device, before initiating a license check test.</li>
</ol>

<p>The sections below provide more information.</p>


<h3 id="test-response">Setting test responses for license checks</h3>

<p>Android Market provides a configuration setting in your publisher account
that lets you override the normal processing of a license check and return a
specified static response code. The setting is for testing only and applies
<em>only</em> to license checks for applications that you have uploaded, made by
any user signed in to an emulator or device using the credentials of the
publisher account or a registered test account. For other users, the server
always processes license checks according to normal rules.  </p>

<p>To set a test response for your account, sign in to your publisher account
and click "Edit Profile". In the Edit Profile page, locate the Test Response
menu in the Licensing panel, shown below. You can select from the full set of
valid server response codes to control the response or condition you want to
test in your application.</p>

<p>In general, you should make sure to test your application's licensing
implementation with every response code available in the Test Response menu.
For a description of the codes, see <a href="#server-response-codes">Server
Response Codes</a> in the Appendix of this document.</p>

<div style="margin-bottom:2em;" id="licensing_test_response">

<img src="../../images/licensing_test_response.png" style="text-align:left;margin-bottom:0;" />
<div style="margin:0 2em;padding:0"><strong>Figure 7.</strong> The Licensing
panel of your account's Edit Profile page, showing the Test Accounts field and the
Test Response menu.</div>
</div>

<p>Note that the test response that you configure applies account-wide &mdash;
that is, it applies not to a single application, but to <em>all</em>
applications associated with the publisher account. If you are testing multiple
applications at once, changing the test response will affect all of those
applications on their next license check (if the user is signed into
the emulator or device using the publisher account or a test account).</p>

<p>Before you can successfully receive a test response for a license check,
you must sign in to the device or emulator on which the application
is installed, and from which it is querying the server. Specifically, you must
sign using either your publisher account or one of the test accounts that you
have set up. For more information about test accounts, see the next section.</p>

<p>See <a href="#server-response-codes">Server Response Codes</a> for a list of
test responses available and their meanings. </p>


<h3 id="test-acct-setup">Setting up test accounts</h3>

<p>In some cases, you might want to let multiple teams of developers test
licensing on applications that will ultimately be published through your
publisher account, but without giving them access to your publisher account's
sign-in credentials. To meet that need, the Android Market publisher site lets
you set up one or more optional <em>test accounts</em> &mdash; accounts that are
authorized to query the licensing server and receive static test responses from
your publisher account.</p>

<p>Test accounts are standard Google accounts that you register on your
publisher account, such that they will receive the test response for
applications that you have uploaded. Developers can then sign in to their
devices or emulators using the test account credentials and initiate license
checks from installed applications. When the licensing server receives a license
check from a user of a test account, it returns the static test response
configured for the publisher account.  </p>

<p>Necessarily, there are limitations on the access and permissions given to
users signed in through test accounts, including:</p>

<ul>
<li>Test account users can query the licensing server only for applications that
are already uploaded to the publisher account. </li>
<li>Test account users do not have permission to upload applications to your
publisher account.</li>
<li>Test account users do not have permission to set the publisher account's
static test response.</li>
</ul>

<p>The table below summarizes the differences in capabilities, between the
publisher account, a test account, and any other account.</p>

<p class="table-caption" id="acct-types-table"><strong>Table 1.</strong>
Differences in account types for testing licensing.</p>

<table>
<tr>
<th>Account Type</th>
<th>Can check license before upload?</th>
<th>Can receive test response?</th>
<th>Can set test response?</th>
</tr>

<tr>
<td>Publisher account</td>
<td>Yes</td>
<td>Yes</td>
<td>Yes</td>
</tr>

<tr>
<td>Test account</td>
<td>No</td>
<td>Yes</td>
<td>No</td>
</tr>

<tr>
<td>Other</td>
<td>No</td>
<td>No</td>
<td>No</td>
</tr>
</table>

<h4 id="reg-test-acct">Registering test accounts on the publisher account</h4>

<p>To get started, you need to register each test account in your publisher
account. As shown in <a href="#licensing_test_response">Figure 7</a>, above, you
register test accounts in the Licensing panel of your publisher account's Edit
Profile page. Simply enter the accounts as a comma-delimited list and click
<strong>Save</strong> to save your profile changes.</p>

<p>You can use any Google account as a test account. If you want to own and
control the test accounts, you can create the accounts yourself and distribute
the credentials to your developers or testers.</p>

<h4 id="test-app-upload">Handling application upload and distribution for test
account users</h4>

<p>As mentioned above, users of test accounts can only receive static test
responses for applications that are uploaded to the publisher account. Since
those users do not have permission to upload applications, as the publisher you
will need to work with those users to collect apps for upload and distribute
uploaded apps for testing. You can handle collection and distribution in any way
that is convenient. </p>

<p>Once an application is uploaded and becomes known to the licensing server,
developers and testers can continue modify the application in their local
development environment, without having to upload new versions. You only need to
upload a new version if the local application increments the
<code>versionCode</code> attribute in the manifest file. </p>

<h4 id="test-key">Distributing your public key to test account users</h4>

<p>The licensing server handles static test responses in the normal way,
including signing the license response data, adding extras parameters, and so
on. To support developers who are implementing licensing using test accounts,
rather than the publisher account, you will need to distribute
your public key to them. Developers without access to the publisher site do not
have access to your public key, and without the key they won't be able to
verify license responses. </p>

<p>Note that if you decide to generate a new licensing key pair for your account
for some reason, you need to notify all users of test accounts. For
testers, you can embed the new key in the application package and distribute it
to users. For developers, you will need to distribute the new key to them
directly. </p>


<h3 id="acct-signin">Signing in to an authorized account in the runtime
environment</h3>

<p>The licensing service is designed to determine whether a given user is
licensed to use a given application &mdash; during a license check, the Android
Market application gathers the user ID from the primary account on the system
and sends it to the server, together with the package name of the application
and other information. However, if there is no user information available, the
license check cannot succeed, so the Android Market application terminates the
request and returns an error to the application. </p>

<p>During testing, to ensure that your application can successfully query the
licensing server, you must make sure that you sign in to an account <em>on the
device or emulator</em> using:</p>

<ul>
<li>The credentials of a publisher account, or</li>
<li>The credentials of a test account that is registered with a publisher
account</li>
</ul>


<div class="sidebox-wrapper">
<div class="sidebox">
<h2>Signing in to a Google account on an emulator</h2>

<p>If you are testing licensing on an emulator, you need to sign in to a Google
account on the emulator. If you do not see an option to create a new Google
account, the problem might be that your AVD is running a standard Android system
image, rather than the Google APIs Add-On, API 8 (release 2) or higher. </p>

<p style="margin-top:.5em;">For more information, see <a
href="#runtime-setup">Setting up the runtime environment</a>, above.</p>

</div>
</div>

<p>Signing in using a publisher account offers the advantage of letting your
applications receive static test responses even before the applications are
uploaded to the publisher site.</p>

<p>If you are part of a larger organization or are working with external groups
on applications that will be published through your site, you will more likely
want to distribute test accounts instead, then use those to sign in during
testing. </p>

<p>To sign in on a device or emulator, follow the steps below. The preferred
approach is to sign in as the primary account &mdash; however, if there are
other accounts already in use on the device or emulator, you can create an
additional account and sign in to it using the publisher or test account
credentials.  </p>

<ol>
<li>Open Settings &gt; Accounts &amp; sync</li>
<li>Select <strong>Add Account</strong> and choose to add a "Google" account.
</li>
<li>Select <strong>Next</strong> and then <strong>Sign in</strong>.</li>
<li>Enter the username and password of either the publisher account or a test
account that is registered in the publisher account.</li>
<li>Select <strong>Sign in</strong>. The system signs you in to the new
account.</li>
</ol>

<p>Once you are signed in, you can begin testing licensing in your application
(if you have completed the LVL integration steps above). When your application
initiates a license check, it will receive a response containing the static test
response configured on the publisher account. </p>

<p>Note that, if you are using an emulator, you will need to sign in to the
publisher account or test account each time you wipe data when restarting the
emulator.</p>

<div style="margin:2em 1em 1em 1em;">

<img src="../../images/licensing_device_signin.png" style="text-align:left;" />
<div style="margin:.25em 1.25em;padding:0"><strong>Figure 8.</strong> Example of
setting up a Google account on a device or emulator.</div>
</div>

<h2 id="app-obfuscation">Obfuscating Your Application</h2>

<p>To ensure the security of your application, particularly for a paid
application that uses licensing and/or custom constraints and protections, it's
very important to obfuscate your application code. Properly obfuscating your
code makes it more difficult for a malicious user to decompile the application's
bytecode, modify it &mdash; such as by removing the license check &mdash;
and then recompile it.</p>

<p>Several obfuscator programs are available for Android applications, including
<a href="http://proguard.sourceforge.net/">ProGuard</a>, which also offers
code-optimization features. The use of ProGuard or a similar program to obfuscate
your code is <em>strongly recommended</em> for all applications that use Android
Market Licensing. </p>

<h2 id="app-publishing">Publishing a Licensed Application</h2>

<p>When you are finished testing your license implementation, you are ready to
publish the application on Android Market. Follow the normal steps to <a
href="../../guide/publishing/preparing.html">prepare</a>, <a
href="../../guide/publishing/app-signing.html">sign</a>, and then <a
href="../../guide/publishing/publishing.html">publish the application</a>.
</p>

<h4>Removing Copy Protection</h4>

<p>After uploading your licensed application, remember to remove copy protection
from the application, if it is currently used. To check and remove copy
protection, sign in to the publisher site and go the application's upload
details page. In the Publishing options section, make sure that the Copy
Protection radio button selection is "Off".</p>

<h4>Considerations for Free Apps</h4>

<p>Licensing is currently supported only for paid applications. If you already
published your application as free, you won't be able to upload an updated
version that includes licensing (that is, an application that uses the same
package name and that includes the <a href="#manifest-permission">licensing
permission</a>). Here are some points to keep in mind:</p>

<ul>
<li>If you want to offer a free version of your application that provides a
reduced feature set (or that offers the full feature set for trial period), the
free version of your application must not include the licensing permission and
must use a different package name than the paid version of the app.</li>
<li>If you want to offer a paid version of your free application that uses
licensing, you can do so under a new package name.</li>
</ul>

<h2 id="support">Where to Get Support</h2>

<p>If you have questions or encounter problems while implementing or deploying
publishing in your applications, please use the support resources listed in the
table below. By directing your queries to the correct forum, you can get the
support you need more quickly. </p>

<p class="table-caption"><strong>Table 2.</strong> Developer support resources
for Android Market Licensing Service.</p>

<table>

<tr>
<th>Support Type</th>
<th>Resource</th>
<th>Range of Topics</th>
</tr>
<tr>
<td rowspan="2">Development and testing issues</td>
<td>Google Groups: <a
href="http://groups.google.com/group/android-developers">android-developers</a>
</td>
<td rowspan="2">LVL download and integration, library projects, Policy
questions, user experience ideas, handling of responses, Obfuscator, IPC, test
environment setup</td>
</tr>
<tr>
<td>Stack Overflow: <a
href="http://stackoverflow.com/questions/tagged/android">http://stackoverflow.com/questions/tagged/android</a></td>
</tr>
<tr>
<td rowspan="2">Accounts, publishing, and deployment issues</td>
<td><a href="http://www.google.com/support/forum/p/Android+Market">Android
Market Help Forum</a></td>
<td rowspan="2">Publisher accounts, licensing key pair, test accounts, server
responses, test responses, application deployment and results</td>
</tr>
<tr>
<td><a
href="http://market.android.com/support/bin/answer.py?answer=186113">Market
Licensing Support FAQ</a></td>
</tr>
<tr>
<td>LVL issue tracker</td>
<td><a href="http://code.google.com/p/marketlicensing/issues/">Marketlicensing
project issue tracker</a></td>
<td>Bug and issue reports related specifically to the LVL source code classes
and interface implementations</td>
</tr>

</table>

<p>For general information about how to post to the groups listed above, see <a
href="../../resources/community-groups.html">Developer Forums</a> document
in the Resources tab.</p>

<h2 id="lvl-summary">Summary of LVL Classes and Interfaces</h2>

<p>The table below lists all of the source files in the License Verification
Library (LVL) available through the Android SDK. All of the files are part of
the <code>com.android.vending.licensing</code> package.</p>

<p class="table-caption"><strong>Table A-1.</strong> Summary of LVL library
classes and interfaces.</p>

<div style="width:99%">
<table width="100%">

<tr>
<th width="15%">Category</th>
<th width="20%">Name</th>
<th width="100%">Description</th>
</tr>

<tr>
<td rowspan="2">License check and result</td>
<td>LicenseChecker</td>
<td>Class that you instantiate (or subclass) to initiate a license check.</td>
</tr>
<tr>
<td><em>LicenseCheckerCallback</em></td>
<td>Interface that you implement to handle result of the license check.</td>
</tr>

<tr>
<td rowspan="3" width="15%">Policy</td>
<td width="20%"><em>Policy</em></td>
<td width="100%">Interface that you implement to determine whether to allow
access to the application, based on the license response. </td>
</tr>
<tr>
<td>ServerManagedPolicy</td>
<td width="100%">Default Policy implementation. Uses settings provided by the
licensing server to manage local storage of license data, license validity,
retry.</td>
</tr>
<tr>
<td>StrictPolicy</td>
<td>Alternative Policy implementation. Enforces licensing based on a direct
license response from the server only. No caching or request retry.</td>
</tr>

<tr>
<td rowspan="2" width="15%">Data obfuscation <br><em>(optional)</em></td>
<td width="20%"><em>Obfuscator</em></td>
<td width="100%">Interface that you implement if you are using a Policy (such as
ServerManagedPolicy) that caches license response data in a persistent store.
Applies an obfuscation algorithm to encode and decode data being written or
read.</td>
</tr>
<tr>
<td>AESObfuscator</td>
<td>Default Obfuscator implementation that uses AES encryption/decryption
algorithm to obfuscate/unobfuscate data.</td>
</tr>

<tr>
<td rowspan="2" width="15%">Device limitation<br><em>(optional)</em></td>
<td width="20%"><em>DeviceLimiter</em></td>
<td width="100%">Interface that you implement if you want to restrict use of an
application to a specific device. Called from LicenseValidator. Implementing
DeviceLimiter is not recommended for most applications because it requires a
backend server and may cause the user to lose access to licensed applications,
unless designed with care.</td>
</tr>
<tr>
<td>NullDeviceLimiter</td>
<td>Default DeviceLimiter implementation that is a no-op (allows access to all
devices).</td>
</tr>

<tr>
<td rowspan="6" width="15%">Library core, no integration needed</td>
<td width="20%">ResponseData</td>
<td width="100%">Class that holds the fields of a license response.</td>
</tr>
<tr>
<td>LicenseValidator</td>
<td>Class that decrypts and verifies a response received from the licensing
server.</td>
</tr>
<tr>
<td>ValidationException</td>
<td>Class that indicates errors that occur when validating the integrity of data
managed by an Obfuscator.</td>
</tr>
<tr>
<td>PreferenceObfuscator</td>
<td>Utility class that writes/reads obfuscated data to the system's
<code><a href="../../reference/android/content/SharedPreferences.html">SharedPreferences</a></code> store.</td>
</tr>
<tr>
<td><em>ILicensingService</em></td>
<td>One-way IPC interface over which a license check request is passed to the
Android Market client.</td>
</tr>
<tr>
<td><em>ILicenseResultListener</em></td>
<td>One-way IPC callback implementation over which the application receives an
asynchronous response from the licensing server.</td>
</tr>

</table>
</div>


<h2 id="server-response-codes">Server Response Codes</h2>

<p>The table below lists all of the license response codes supported by the
licensing server. In general, an application should handle all of these response
codes. By default, the LicenseValidator class in the LVL provides all of the
necessary handling of these response codes for you. </p>

<p class="table-caption"><strong>Table A-2.</strong> Summary of response codes
returned by the Android Market server in a license response.</p>

<table>

<tr>
<th>Response Code</th>
<th>Description</th>
<th>Signed?</th>
<th>Extras</th>
<th>Comments</th>
</tr>
<tr>
<td>LICENSED</td>
<td>The application is licensed to the user. The user has purchased the
application or the application only exists as a draft.</td>
<td>Yes</td>
<td><code>VT</code>,&nbsp;<code>GT</code>, <code>GR</code></td>
<td><em>Allow access according to Policy constraints.</em></td>
</tr>
<tr>
<td>LICENSED_OLD_KEY</td>
<td>The application is licensed to the user, but there is an updated application
version available that is signed with a different key. </td>
<td>Yes </td>
<td><code>VT</code>, <code>GT</code>, <code>GR</code>, <code>UT</code></td>
<td><em>Optionally allow access according to Policy constraints.</em>
<p style="margin-top:.5em;">Can indicate that the key pair used by the installed
application version is invalid or compromised. The application can allow access
if needed or inform the user that an upgrade is available and limit further use
until upgrade.</p>
</td>
</tr>
<tr>
<td>NOT_LICENSED</td>
<td>The application is not licensed to the user.</td>
<td>No</td>
<td></td>
<td><em>Do not allow access.</em></td>
</tr>
<tr>
<td>ERROR_CONTACTING_SERVER</td>
<td>Local error &mdash; the Android Market application was not able to reach the
licensing server, possibly because of network availability problems. </td>
<td>No</td>
<td></td>
<td><em>Retry the license check according to Policy retry limits.</em></td>
</tr>
<tr>
<td>ERROR_SERVER_FAILURE</td>
<td>Server error &mdash; the server could not load the publisher account's key
pair for licensing.</td>
<td>No</td>
<td></td>
<td><em>Retry the license check according to Policy retry limits.</em>
</td>
</tr>
<tr>
<td>ERROR_INVALID_PACKAGE_NAME</td>
<td>Local error &mdash; the application requested a license check for a package
that is not installed on the device. </td>
<td>No </td>
<td></td>
<td><em>Do not retry the license check.</em>
<p style="margin-top:.5em;">Typically caused by a development error.</p>
</td>
</tr>
<tr>
<td>ERROR_NON_MATCHING_UID</td>
<td>Local error &mdash; the application requested a license check for a package
whose UID (package, user ID pair) does not match that of the requesting
application. </td>
<td>No </td>
<td></td>
<td><em>Do not retry the license check.</em>
<p style="margin-top:.5em;">Typically caused by a development error.</p>
</td>
</tr>
<tr>
<td>ERROR_NOT_MARKET_MANAGED</td>
<td>Server error &mdash; the application (package name) was not recognized by
Android Market. </td>
<td>No</td>
<td></td>
<td><em>Do not retry the license check.</em>
<p style="margin-top:.5em;">Can indicate that the application was not published
through Android Market or that there is an development error in the licensing
implementation.</p>
</td>
</tr>

</table>

<p class="note"><strong>Note:</strong> As documented in <a href="#test-env">
Setting Up The Testing Environment</a>, the response code can be manually
overridden for the application developer and any registered test users via the
Android Market publisher site.
<br/><br/>
Additionally, as noted above, applications that are in draft mode (in other
words, applicaitons that have been uploaded but have <em>never</em> been
published) will return LICENSED for all users, even if not listed as a test
user. Since the application has never been offered for download, it is assumed
that any users running it must have obtained it from an authorized channel for
testing purposes.</p>

<h2 id="extras">Server Response Extras</h2>

<p>The licensing server includes several settings in certain types of license
responses, to assist the application and its Policy in managing access to the
application across the 24-hour refund period and other conditions. Specifically,
the server provides recommended values for the application's license validity
period, retry grace period, maximum allowable retry count, and other settings.
The server appends the settings as key-value pairs in the license response
"extras" field. </p>

<p>Any Policy implementation can extract the extras settings from the license
response and use them as needed. The LVL default Policy implementation, <a
href="#ServerManagedPolicy">ServerManagedPolicy</a>, serves as a working
implementation and an illustration of how to obtain, store, and use the
settings. </p>

<p class="table-caption"><strong>Table A-3.</strong> Summary of
license-management settings supplied by the Android Market server in a license
response.</p>

<table>
<tr>
<th>Extra</th><th>Description</th>
</tr>

<tr>
  <td>VT</td>
  <td>License validity timestamp. Specifies the date/time at which the current
(cached) license response expires and must be rechecked on the licensing server.
 </td>
</tr>
<tr>
  <td>GT</td>
  <td>Grace period timestamp. Specifies the end of the period during which a
Policy may allow access to the application, even though the response status is
RETRY. <p>The value is managed by the server, however a typical value would be 5
or more days.</p></td>
</tr>
<tr>
  <td>GR</td>
  <td>Maximum retries count. Specifies how many consecutive RETRY license checks
the Policy should allow, before denying the user access to the application.
<p>The value is managed by the server, however a typical value would be "10" or
higher.</p></td>
</tr>
<tr>
  <td>UT</td>
  <td>Update timestamp. Specifies the day/time when the most recent update to
this application was uploaded and published. <p>The server returns this extra
only for LICENSED_OLD_KEYS responses, to allow the Policy to determine how much
time has elapsed since an update was published with new licensing keys before
denying the user access to the application. </p></td>
</tr>

</table>

<p>The sections below provide more information about the server-provided
settings and how to use them. </p>

<h4>License validity period</h4>

<p>The Android Market licensing server sets a license validity period for all
downloaded applications. The period expresses the interval of time over which an
application's license status should be considered as unchanging and cacheable by
a licensing Policy in the application. The licensing server includes the
validity period in its response to all license checks, appending an
end-of-validity timestamp to the response as an extra under the key "VT". A
Policy can extract the VT key value and use it to conditionally allow access to
the application without rechecking the license, until the validity period
expires. </p>

<p>The license validity signals to a licensing Policy when it must recheck the
licensing status with the licensing server. It is <em>not</em> intended to imply
whether an application is actually licensed for use. That is, when an
application's license validity period expires, this does not mean that the
application is no longer licensed for use &mdash; rather, it indicates only that
the Policy must recheck the licensing status with the server. It follows that,
as long as the license validity period is not expired, it is acceptable for the
Policy to cache the initial license status locally and return the cached license
status instead of sending a new license check to the server.</p>

<p>The licensing server manages the validity period as a means of helping the
application properly enforce licensing across the refund period offered by
Android Market for paid applications. It sets the validity period based on
whether the application was purchased and, if so, how long ago. Specifically,
the server sets a validity period as follows:</p>

<ul>
<li>For a paid application, the server sets the initial license validity period
so that the license response remains valid for as long as the application is
refundable. A licensing Policy in the application may cache the
result of the initial license check and does not need to recheck the license
until the validity period has expired.</li>
<li>When an application is no longer refundable, the server
sets a longer validity period &mdash; typically a number of days. </li>
<li>For a free application, the server sets the validity period to a very high
value (<code>long.MAX_VALUE</code>). This ensures that, provided the Policy has
cached the validity timestamp locally, it will not need to recheck the
license status of the application in the future.</li>
</ul>

<p>The ServerManagedPolicy implementation uses the extracted timestamp
(<code>mValidityTimestamp</code>) as a primary condition for determining whether
to recheck the license status with the server before allowing the user access to
the application. </p>

<h4>Retry period and maximum retry count</h4>

<p>In some cases, system or network conditions can prevent an application's
license check from reaching the licensing server, or prevent the server's
response from reaching the Android Market client application. For example, the
user might launch an application when there is no cell network or data
connection available &mdash; such as when on an airplane &mdash; or when the
network connection is unstable or the cell signal is weak. </p>

<p>When network problems prevent or interrupt a license check, the Android
Market client notifies the application by returning a "RETRY" response code to
the Policy's <code>processServerResponse()</code> method. In the case of system
problems, such as when the application is unable to bind with Android Market's
ILicensingService implementation, the LicenseChecker library itself calls the
Policy <code>processServerResonse()</code> method with a "RETRY" response code.
</p>

<p>In general, the RETRY response code is a signal to the application that an
error has occurred that has prevented a license check from completing. 

<p>The Android Market server helps an application to manage licensing under
error conditions by setting a retry "grace period" and a recommended maximum
retries count. The server includes these values in all license check responses,
appending them as extras under the keys "GT" and "GR". </p>

<p>The application Policy can extract the GT and GR extras and use them to
conditionally allow access to the application, as follows:</p>

<ul>
<li>For a license check that results in a RETRY response, the Policy should
cache the RETRY response code and increment a count of RETRY responses.</li>
<li>The Policy should allow the user to access the application, provided that
either the retry grace period is still active or the maximum retries count has
not been reached.</li>
</ul>

<p>The ServerManagedPolicy uses the server-supplied GT and GR values as
described above. The example below shows the conditional handling of the retry
responses in the <code>allow()</code> method. The count of RETRY responses is
maintained in the <code>processServerResponse()</code> method, not shown. </p>


<pre>    public boolean allowAccess() {
        long ts = System.currentTimeMillis();
        if (mLastResponse == LicenseResponse.LICENSED) {
            // Check if the LICENSED response occurred within the validity timeout.
            if (ts &lt;= mValidityTimestamp) {
                // Cached LICENSED response is still valid.
                return true;
            }
        } else if (mLastResponse == LicenseResponse.RETRY &amp;&amp;
                   ts &lt; mLastResponseTime + MILLIS_PER_MINUTE) {
            // Only allow access if we are within the retry period or we haven't used up our
            // max retries.
            return (ts &lt;= mRetryUntil || mRetryCount &lt;= mMaxRetries);
        }
        return false;
    }</pre>


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